Tiny fluctuations in the intensity of the
→ cosmic microwave background radiation (CMBR)
as a function of angular position over the sky, first discovered in the
→ Cosmic Background Explorer (COBE)
observations. At a level of 1 part in 100,000, these temperature
variations trace the distribution of matter and energy when the Universe was very
young, about 380,000 years old.
Since the CMB spectrum is described to a high precision by a
→ blackbody
law with temperature T0, it is usual to express
the anisotropies in terms of temperature fluctuations ΔT/T0
and expand them on the sky in → spherical harmonic
series
ΔT/T0 (θ,φ) = Σ
almYlm(θ,φ), where
θ and φ are the → spherical polar coordinates,
Ylm is the spherical harmonic functions with
→ multipole indexl, and
the sum runs over l = 1, 2, ..., ∞, m = -l, ..., l, giving 2l + 1
values of m for each l, and alm is the
multipole moment of the decomposition.
The power spectrum of the anisotropies is defined as
Cl≡ mean | alm |2 = 1/(2l + 1)
Σ mean | alm |2.
See also → CMB angular power spectrum.

Thermal radiation with a temperature of 2.73 K that is apparently
uniformly distributed in the Universe. It is believed to be a redshifted
remnant of the hot radiation that was in thermal equilibrium with
matter during the first hundred thousand years after the Big Bang.
Same as → cosmic microwave background (CMB) radiation.

A space telescope launched by NASA in 2001 which measures the
temperature fluctuations in the
→ cosmic microwave background (CMB) radiation.
It creates a full-sky map of the CMB, with a 13 arcminute resolution
via multi-frequency observations. WMAP is the first mission to use a
→ Lagrangian point L2 as its permanent
observing station at a distance of 1.5 million km. WMAP completed its
prime two years of mission operations in September 2003 and is
continuing in 2009 its observations for still several years to come. WMAP's
measurements have played a considerable role in establishing the current
standard model of cosmology. They are consistent with a Universe that is
dominated by → dark energy, with negative pressure or a
→ cosmological constant. In this model, the age of the Universe
is 13.73 ± 0.12 billion years. The current expansion rate of the Universe measured by
the Hubble constant, is 70.5 ± 1.3 km·s-1 Mpc-1.
The content of the Universe consists of 4.56% ± 0.15% ordinary
→ baryonic matter,
22.8% ± 1.3% → cold dark matter,
and 72.6% ± 1.5% of → dark energy,
that accelerates the → expansion of the Universe.

WMAP, short for Wilkinson Microwave Anisotropy Probe, in
honor of David Todd Wilkinson (1935-2002), who had been a member of the mission's
science team.